Prosaposin, a 70 kilodalton glycoprotein, is the precursor of a group of four small heat-stable glycoproteins which are required for hydrolysis of glycosphingolipids by lysosomal hydrolases (Kishimoto et al., (1992) J. Lipid Res., 33: 1255-1267) Prosaposin is proteolytically processed in lysosomes to generate saposins A, B, C, and D which exist as four adjacent tandem domains in prosaposin (O'Brien and Kishimoto, (1991) FASEB J., 5: 301-308) All four saposins are structurally similar to each other, including the placement of six cysteines, a glycosylation site and conserved proline residues.
Unprocessed prosaposin also exists as an integral membrane protein and a secreted protein which is present in human milk, cerebrospinal fluid and seminal plasma. The presence of high concentrations of unprocessed prosaposin in the central nervous system indicates that it may play a significant role in addition to activation of lysosomal hydrolases.
Prosaposin binds membrane lipids called glycosphingolipids which are sphingolipids consisting of a carbohydrate head group and two hydrocarbon chains; a fatty acid and a sphingosine derivative. Glycosphingolipids are important components of the myelin sheath, a structure which protects and insulates nerve fibers. Demyelination is a defect common to a number of central nervous system disorders, the most common being multiple sclerosis (MS). MS, a chronic disorder which may lead to total disability, is characterized by damage to the myelin sheath leaving the axons mostly intact. It is currently believed that autoimmune mechanisms, perhaps virally-induced, may play a role in development of the disease. There is currently no effective treatment for MS. Other central nervous system disorders involving demyelination include acute disseminated encephalomyelitis, amyotrophic lateral sclerosis, acute necrotizing hemorrhagic leukodystrophy, progressive multifocal leukoencephalitis, metachromatic leukodystrophy and adrenal leukodystrophy. An example of a demyelinating disease of the peripheral nervous system is Guillain-Barré syndrome (Pathologic Basis of Disease, Robbins, S. L. and Cotran, R. S., eds, W.B. Saunders, Philadelphia, (1979), pp. 1578-1582).
Post-polio syndrome is characterized by muscle fatigue and decreased endurance with accompanying muscle weakness and atrophy. The disease is believed to be caused in part by the same type of spinal cord motor neurons that are damaged in amyotrophic lateral sclerosis.
Peripheral nerve injuries and peripheral neuropathies, such as those resulting from diabetes or chemotherapy, comprise the most prevalent peripheral nervous system disorders (see Table 1) Current treatments for peripheral nerve disorders only treat the symptoms, not the cause of the disease.
TABLE 1DiseaseNo. of U.S. patientsAmyotrophic Lateral30,000SclerosisSpinal Muscular Atrophy50,000Post-Polio Syndrome250,000 Guillain-Barré Syndrome20,000Muscular Dystrophies175,000 Peripheral Neuropathies1,000,000  Peripheral Nerve Injuries500,000 Total2,150,000  
Prosaposin binds glycosphingolipids such as gangliosides, cerebrosides and sulfatides with high affinity and facilitates their transfer from micelles to membranes (Sueda, et al. (1993) J. Biol. Chem. in press; Hiraiwa et al., (1992) Proc. Natl. Acad. Sci. USA., 89: 11254-11258). Gangliosides contain one or more sialic acid residues and are most abundant in the plasma membrane of neurons where they constitute approximately 6% of the total lipid mass. Although the function of gangliosides is largely unknown, they have been implicated in the stimulation of neuronal differentiation, neuritogenesis and nervous system repair.
Neurotrophins may be defined as those proteins capable of affecting the survival, target innervation and/or function of neuronal cell populations (Barde, (1989) Neuron, 2: 1525-1534). The efficacy of neurotrophins both in vitro and in vivo has been well-documented. The most well-characterized of such proteins is nerve growth factor (NGF) which is synthesized by target cells of sympathetic and sensory neurons and acts as a trophic factor for forebrain cholinergic, peripheral and sensory neurons (Hefti et al., (1989) Neurobiol. Aging, 10: 515-533). In vivo experiments indicate that NGF can reverse naturally-occurring as well as physical traumatic injuries to peripheral nerves. For example, local application of NGF has been shown to prevent the atrophy of sensory ganglia resulting from transection of the sciatic nerve in adult rats (Rich et al., (1987) J. Neurocytol., 16: 261-268). In addition, NGF plays a role in the neural regenerative process since it enhances neurite extension of developing sympathetic and sensory neurons (Purves et al., (1988) Nature, 336: 123-128). Moreover, since NGF supports the function of forebrain cholinergic neurons which are lost in Alzheimer's patients, this indicates that NGF may have a clinical use in treatment of this disease (Hefti et al., (1989) Neurobiol. Aging, 10: 515-533).
Brain-Derived Neurotrophic Factor (BDNF) is synthesized in the central nervous system and is a trophic factor for peripheral sensory neurons, dopaminergic neurons of the substantia nigra, central cholinergic neurons and retinal ganglia (Henderson et al., (1993) Restor. Neurol. Neurosci., 5: 15-28). BDNF has also been shown to prevent normally-occurring cell death both in vitro and in vivo (Hofer and Barde, (1988) Nature, 331: 261-262).
Since NGF and BDNF share large regions of homology (approximately 50%), degenerate oligonucleotide primers corresponding to four of these regions were used in PCR reactions to amplify novel related sequences. A related neurotrophic factor called neurotrophin 3 (NT-3) was cloned (Maisonpierre et al., (1990) Science, 247: 1446-1451). NT-3 is found both centrally and peripherally and is capable of promoting survival of sensory and sympathetic neurons, including dorsal root ganglia (DRG) explants.
The three neurotrophins described above have different neuronal specificities. All three neurotrophins induced neurite outgrowth from DRG explants. NGF induces neurite outgrowth from sympathetic ganglia (SG) but not nodose ganglion (NG), whereas BDNF induces neurite outgrowth from NG but not SG. NT-3 promotes neurite outgrowth from NG and to a lesser extent from SG, suggesting a broader specificity than either NGF or BDNF (Lindsay et al., (1991) Restor. Neurol. Neurosci., 2: 211-220).
Ciliary Neurotrophic Factor (CNTF) promotes survival of chicken embryo ciliary ganglia in vitro and was also found to support survival of cultured sympathetic, sensory and spinal motor neurons (Ip et al., (1991) J. Physiol., Paris, 85: 123-130). Local administration of this protein to the lesion site of newborn rats has been shown to prevent the degeneration of the corresponding motor neurons. CNTF also rescued motor neurons from developmental cell death (Henderson et al., (1993) Restor. Neurol. Neurosci., 5: 15-28).
Fibroblast Growth Factor (FGF) can also promote in vitro survival of embryonic neurons. Effects have also been observed on neuronal survival after lesion in vivo. FGF apparently acts on a wide variety of neurons (Hefti et al., (1989) Neurobiol. Aging, 10: 515-533).
The identification of prosaposin itself as a neurotrophic factor which is present in the cell bodies of large populations of neurons including upper and lower motor neurons, and its ability to induce myelination in mouse cerebellar explants, represent significant new functions for this protein. Additionally, the fact that fragments of prosaposin retain neurotogenic activity has no precedent in the literature. No reports have appeared on the use of small active fragments of the aforementioned neurotrophic factors to promote neuronal survival and differentiation. Prosaposin and its derivatives are therefore believed to have important therapeutic potential in the treatment of neurodegenerative and demyelination disorders.